Yeah, that's interesting, huh?
So far as I can tell there is nothing in Christian theology that says anything about there being only "one true soul mate"; the only place I can actually find it is in Judaism, with the concept of bashert. No other religion has this "there is only one soul mate for everyone" belief (I'm assuming your friend is Christian, not Jewish; if he's Jewish, then yeah, it's just a part of his belief system). Yet it seems to have entered popular culture regardless, and formed its own kind of romantic religion.
I'll do an article on it!